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London police released a computer-generated likeness of a man seen near the scene of the double murders of two French students as a 21-year-old man arrested in connection with the brutal homicides was released.

The parents of one of the two French students murdered in London a week ago in a frenzied knife attack appealed on Sunday for the killer to come forward and said they would not rest until the guilty party was found.

Bio-engineering students Laurent Bonomo and Gabriel Ferez, both 23, were bound, gagged and repeatedly stabbed in Bonomo's south London apartment in what police said were the most "frenzied, brutal and horrific" murders they had seen.

French newspapers said the shocking murders highlighted how dangerous the city had become after a spate of street teen stabbings this year that has prompted the mayor to make it a priority and police to expand a specialist knife crime squad.

"We plead with you to turn yourselves in to the police. You will not be able to live in hiding forever," Ferez' parents Francoise and Olivier said in a statement.

"You may be scared and feel like a coward, but you must recognise this terrible mistake you made. Rest assured that we will not leave you in peace."

Late on Sunday police issued a computer-generated likeness of a man seen running away from near the apartment on the night of the murders.

Witnesses described him as white, aged between 30 and 40, of slim build and wearing a light coloured baseball cap, dark top, blue jeans and white trainers.

Mick Duthie, the man leading the police inquiry, urged that anyone who may have seen something to come forward.

Police arrested a 21-year-old man on Saturday in connection with the murders who was released without charge on Sunday.

Bonomo was stabbed nearly 200 times, up to 80 of the wounds inflicted after he was dead, while Ferez received nearly 50 knife wounds.

The killer or killers then set fire to the apartment in what police said was an apparent attempt to destroy evidence.

A black Packard Bell laptop computer was stolen from the flat six days before the murders. Bonomo had disturbed the intruder who stole the laptop.

Forensic experts have found no evidence of forced entry after Sunday's murders, suggesting a set of keys might also have been stolen to gain entry on the day of the killings.

Two Sony handheld games consoles were also taken that day. Credit cards are also believed to be missing.

Police have received about 25 calls from the public after appealing for witnesses.

"Someone, somewhere must know something that will help to solve this terrible crime. We ask everyone to be vigilant against crime; but especially those who live in the area of London where our son died," Ferez' parents said. Already this year 18 teenagers have been knifed to death on London's streets. Police have arrested more than 1,200 people and seized 528 knives during a six-week crackdown sparked by the growing number of teenage stabbings.